At the children's homes, researchers measured levels of volatile organic compounds, or VOCs, which are fumes that can be found in items such as cleaning products, paint, furnishings, polishes, and room fresheners. These fumes can irritate the airways causing narrowing and difficulty breathing.

The researchers also recorded indoor temperature and household humidity, which can affect asthma-triggering dust mites. Some children with asthma have "allergic asthma" meaning that allergens such as household mites or mold can make symptoms worse. Measurements were taken twice -- once in winter and once in summer -- to account for seasonal changes.

Levels of indoor VOCs were significantly higher in the children with asthma. Three VOCs in particular stood out: benzene, which had the highest risk, followed by ethylbenzene and toluene.

"For every 10-unit increase in the concentrations of toluene and benzene, the risk of having asthma increased by almost two and three times, respectively," write the researchers in the journal Thorax.

"Domestic exposure to VOCs at levels [even] below currently accepted recommendations may increase the risk of childhood asthma," write the researchers.

It's not that easy to get VOCs out of your house because they're found in many household products. VOCs may also be embedded in the house itself as part of the paint, flooring, or furniture.

Continued

Scanning ingredient labels might not help, either. New products with different combinations of VOCs come out all the time, making research more complicated.

Many of the children with asthma had significant asthma risk factors besides VOCs: 77% had at least one parent with an allergy problem and 57% had at least one parent with asthma.

Seventy-seven percent of the asthmatic children had a genetic tendency to have allergic reactions such as asthma or allergies; 51% of the children without asthma had this tendency.

Indoor Heaters Strongly Linked to Asthma

Guy Marks, honorary associate professor of respiratory medicine at Australia's University of Sydney, and colleagues conducted the second study, also published in the journal Thorax

Marks looked at connections between early-childhood exposure to fume-emitting indoor heaters and the risks of developing asthma, wheezing, and airway hyperresponsiveness -- a feature of asthma consistent with increased sensitivity of the airways to triggers.

In the Australian town of Belmont, 627 children aged 8 to 11 were recruited for the study.

A "substantial minority" had lived in houses with fume-emitting heaters, which included mainly nonfueled gas heaters, open fire, kerosene heaters, and wood stoves. A fourth of the kids also had mothers who smoked at some point during the children's lives.

The study shows a strong association between the use of fume-emitting heaters during the first year of life and lung problems in children aged 8-11.

Children who had lived in a home with a fume-emitting heater during the first year of their life were 47% more likely to have hyperresponsive airways, wheezing, and had twice the risk of developing asthma compared with children not exposed to fume-emitting heaters early in life.

It didn't seem to matter if exposure happened later in life; the first year appeared to be the crucial time frame.

The association may be due to nitrogen dioxide given off by the heaters, researchers say.

If other studies confirm the findings, the researchers say it might be wise to review heating sources in homes where babies live.